部分除荷法による遅れ破壊き裂発生強さの向上

Abstract

The effect of two types of prestressing, i.e. the partial unloading and the perfect unloading, on the delayed failure strength of the specimens with various thickness and notch root radius has been investigated on JIS SNCM8 steel quenched and tempered at 200°C. Here the partial unloading means that the applied stress is raised to a maximum stress, σmax, then decreased to a testing stress σ, and the perfect unloading means that the stress is once decreased to zero from σmax and is again increased to a testing stress σ(<σmax).Both prestress methods, especially the partial unloading method, can largely increase the 100hr delayed failure strength regardless of specimen thickness and notch root radius if the unloading stress σa(=σmax-σ) is chosen appropriately. In order to raise the 100hr delayed failure strength by both prestress methods, it is necessary to increase the unloading stress to a critical value, (σa)c, which increases with increase of specimen thickness and decrease of notch root radius. The reason why the delayed failure strength is increased by both prestress methods can be explained by the decrease of surface stress at notch root, which will suppress the corrosion reaction and prevent the invasion of hydrogen atoms into the material.